1. Field of the Invention
The present invention relates to an improvement in a sealed lead-acid battery suitable for use as a power supply for portable devices.
2. The Prior Art and Its Problems
Most sealed lead-acid batteries found on the market today as power supplies for portable devices are of two major types: in one type, the container is filled with the minimum required amount of electrolyte, which is retained between electrodes and absorbed by a highly retentive separator (retainer mat) so as to eliminate any flow of the electrolyte; in the second type, the electrolyte is immobilized by gelation with a silica powder. These sealed lead-acid batteries operate by utilizing a phenomenon generally referred to as the "oxygen cycle" in which oxygen gas evolving at the positive plates by electrolysis of water during charging reacts at the negative plates with the charged active material and electrolyte to be converted into water.
The container of such sealed lead-acid batteries has to satisfy the following performance requirements:
(1) It must retain in its interior the positive and negative plates, separator and dilute sulfuric acid electrolyte without letting the electrolyte leak out; the inner surface of the container must be resistant to sulfuric acid;
(2) The interior of the sealed lead-acid battery must be evacuated to a pressure of about one half an atmosphere during normal use, and when overcharged, the gases generated by hydrolysis of water produce a pressure equal to the working pressure of the safety valve; the battery container must have sufficient mechanical strength to withstand such pressure cycles, as well as the impact exerted by such external shocks as drops and collisions;
(3) When atmospheric oxygen enters the sealed lead-acid battery operating on the oxygen cycle principle, oxidation at the negative plates proceeds in preference to conversion to water, thereby upsetting the gas balance in the battery to cause a decrease in the water content of the electrolyte; in order to avoid this problem which will deteriorate the performance of the sealed lead-acid battery using an immobilized electrolyte, the container must have adequately low moisture and oxygen gas permeabilities.
(4) In the sealed lead-acid battery having the immobilized electrolyte, an assembled element must be kept under a highly pressed state and the electrolyte must be closely contacted to the positive and negative active materials in order to conduct the electrochemical reactions, such as charging and discharging, in a stable manner. Most of the prior art sealed lead-acid batteries employ containers of box shape molded from ABS and other resins and in order to satisfy the performance requirements described above, the container must have a wall thickness of about 2 mm.
A typical example of a sealed lead-acid battery fabricated according to the prior art is described hereinafter with reference to FIG. 4. An assembled element composed of a positive plate 8, a negative plate 9 and a separator 10 is accommodated in a container 11 in box form molded from ABS or other resins. The container 11 is sealed by bonding a cover 12 having a vent hole 16, terminal channels 15, etc. to the open end of the container. The battery shown in FIG. 4 is also furnished with a safety valve holder 14
In order to fabricate a sealed lead-acid battery having the construction described above, the assembled element has to be pressed into the opening in the container, but this causes great difficulty when the element is elongated or flat. Furthermore, the friction between each of the electrode plates and the inner surface of the container will cause shedding or cracking of the active materials (the powders of lead oxide and spongy lead) or other potential risks that lead to shorting or shorter life of the battery. The conventional practice of bonding the cover to the battery container with adhesives typically based on urethane or epoxy resins causes another problem in that batteries in process have to stay on the production line until the adhesive cures to a satisfactory degree.
There is a growing demand in the battery industry for smaller, thinner and higher-capacity sealed lead-acid batteries, but the need to employ containers having wall thicknesses of about 2 mm has been a great obstacle to the fabrication of batteries with higher performance.
With a view to solving the problems associated with the prior art sealed lead-acid batteries in such aspects as performance, ease of manufacture and consistency in product quality, various battery designs have been proposed, including a structure in which an assembled element is fitted in a thermoplastic synthetic resin container in frame form and both surfaces of the container are sealed with a synthetic resin film, and a structure in which the assembled element is enveloped with a container solely made of a film. While these designs have their own advantages, they are still unsatisfactory for large-scale production purposes in terms of such aspects as reliability and consistency in manufacture.